Process for producing metals



United States Patent 3,099,554 PRQCESS FOR PRODUCHNG METALS James H.Downing, Bufialo, and Benjamin J. Wilson,

Kenmore, N31 assignors to Union Carbide (Iorporation, a corporation ofNew York No Drawing. Filed Feb. 21, 1961, Ser. No. 90,625 2 Claims. (Cl.75-80) The present invention relates to a process for producingmetals'from oxidic source materials, and in particular, relates to aprocess for producing substantially pure metal by vaporization of themetal from the metal borides.

It has been known for some period of time that nu merous metals can bevolatized from mixtures of different metals. Since the rate ofevaporation of these metals depends upon the temperature to some extent,it is commonly known that higher temperatures will increase the rate ofvaporization. However, the extent the ternperature can be increased islimited by the melting point of the metals themselves when high yieldand high vaporization rates are desired. When the melting point isreached a drastic reduction in the vaporization surface occurs and yieldand vaporization rates decrease.

It is known to distill manganese from its carbon-containing alloys suchas ferromanganese and silicomanganese. According to the prior artprocesses, these alloys are heated in a retort at reduced pressure wherethe easily vaporized manganese evaporates. One of the disadvantages ofthe prior art methods is the necessity of advancing the temperature inmultiple stages to avoid melting the charge and thus greatly reducingthe rate of evaporation owing to the formation of the liquid phase andthe subsequent reduction in evaporating surface. By advancing thetemperature in several stages, a porous graphitic residue is formedwhich functions as a wick and thus preserves the maximum evaporatingsurface.

It is an object of the present invention to provide a process forproducing substantially pure metal from their oxides via a vaporizationprocess.

It is another object to provide a process for producing substantiallypure metals from borides of the metals.

It is a further object to provide a process for the va porization ofmetals from their borides at increased rates of metal production and athigh metal yields.

It is a still further object of the present invention to provide aprocess for vaporizing metals directly from the solid state attemperatures Well in excess of the melting point without fusion of themetal in the boride and consequently at increased rates of production.

Other objects are apparent from the remaining disclosure and appendedclaims.

The process which satisfies the above mentioned objects comprisescharging in the solid state at least one metallic boride into a vacuumfurnace, the metal of said boride being selected from the groupconsisting of manganese, and chromium; evacuating substantially all thegas from the vacuum furnace which is reactive with the boride and theselected metal; heating the boride in vacuo to a temperaturesufficiently high to produce metal vapor at least at a vapor pressure inexcess of the ambient pressure within the vacuum furnace and maintainingthe temperature sufliciently high to maintain the metal vapor at leastat a vapor pressure in excess of the ambient pressure within the vacuumfurnace; and conducting at least a portion of the metal vapor to acondensing zone to be condensed to recover substantially pure metal.

The metals amenable for treatment as borides in the present process aremanganese, and chromium. All these Patented July 30, 1963 metals arecapable of forming borides which are substantially stable :at elevatedtemperatures. The borides of these metals will not detrimeutally breakdown at elevated temperatures before the metal itself can be vaporizedand essentially will not break down at temperatures lower than themelting point of the selected metals in the borides themselves. That is,the borides are essentially stable at least up to the melting point ofthe metals in the boride and preferably are substantially stable attemperatures considerably in excess of the melting point of the metalcontained therein.

The borides of the above metals can be produced by a variety of knownmetallurgical processes. For example, when treating oxidic manganesesources such as ore or slag, a co-reduction of oxidic manganese materialand boron oxide by a carbonaceous reducing agent may be conducted inelectric are or induction furnaces. The reaction may be represented asfollows:

The present novel process may then be utilized to treat the boride ofmanganese to produce manganese metal.

The particle size of the charge is maintained within the range of fromapproximately A3 to /2 inch, the preferred size being inch. Theseparticles of the charge may be either solid pieces or pellets composedof compacted fines. The fines may be any size substantially less thaninch but it is to be understood that when finely divided material isused it is to be compacted into pellets approximately A; to /2 inchdiameter.

Any suitable vacuum furnace may be utilized in the present process.

Substantially all the gas which is reactive with the metal boride beingtreated and/ or the metal vapor to be produced must be evacuated fromthe vacuum furnace in practicing the present process. This isaccomplished in particular to avoid production of contaminated metaland/ or to prevent consumption of the borides as well as to obtain therequired vacuum conditions. It is to be noted that the terms vacuum andthe phrase in vacuo are utilized throughout this disclosure and in theclaims to indicate pressure below one standard atmosphere. Specificvacuum pressures will be denoted by millimeters of mercury-(cg. mm. ofHg).

One of the major advantages of the present process is the ability toheat the boride to temperatures considerably in excess of the meltingpoint of the metal constituent in the boride without formation of theliquid phase of the metal. In this manner maximum vaporization rates aremaintained without loss of rate or yield due to the formation ofsubstantial amounts of liquid. It is generally found that the boride inits solid state is heated at as fast a rate of temperature rise aspossible, without causing fusion due to thermal shock, to temperaturessufliciently high to produce vaporization of the metal in the boride.The temperature is maintained at a sufiiciently high level to maintain avapor of the metal at a vapor pressure in excess of the ambient pressurein the vacuum furnace. Note that the metal vapor pressure must be inexcess of the ambient pressure of the furnace to enable an artisan tosecure high rates and yields of the metal sought. The highest possibletemperatures are. preferred up to the temperature immediately below thatlevel which may cause conversion of the metal and/ or boride to theliquid phase.

It is readily apparent that the rate of metal production and to someextent the yield of the metal sought is largely governed by thedifference in the ambient pressure of the atmosphere within the furnaceand the vapor pressure of the metal being volatized. Consequently,extremely low vacuums are most desirable in the present process and,indeed, as low an ambient pressure within the furnace as can bemaintained 'by the equipment should be utilized.

Though the present process is desirably conducted in the above manner,substantial rates and yields of metal can be achieved Where lessdivergent metal vapor and ambient pressures are utilized. This islargely due to the amenability of the borides of the present process totreatment at temperatures far in excess of the melting point of theparticular metallic constituent sought to be recovered from the boride.In the production of manganese from manganese boride a temperature ofabout 1750" C. and an ambient pressure of about 0.05 to 0.07 mm. Hg havebeen found to be satisfactory.

Condensation may be carried out in any one of a variety of knowncondensers. Condensation may be conducted in vacuum or in inert gasatmospheres.

Boron is produced as a byproduct of the present process in the form of aresidue upon vaporization of the metal. The boron may be recycled in theprocess to provide additional borides for treatment.

In the following example an induction heated carbon type vacuum furnacewas utilized.

Manganese boride comprising 76.1 weight percent manganese, 16.9 weightpercent boron, 2.8 Weight percent iron, 2.0 weight percent carbon, 0.05weight percent aluminum and 0.69 weight percent silicon was pelletizedto form compacts approximately /8 to /2 inch in diameter and placed in agraphite crucible and heated in vacuo in a furnace from whichsubstantially all the air had been removed. The rate of temperature risewas about 300 to 400 C. per hour. The heating was continued until atemperature of 1750 C. and a vacuum of about .045 to .090 mm. Hg wasobtained. This temperature was maintained for a period of about 025'hour. The vapors were conducted to a condensing zone and condensed toform manganese metal comprising 98.5 Weight percent manganese. Theresidue was comprised of 65 weight percent boron, 2.87 weight percentmanganese and 9.84 weight percent carbon.

The following example illustrates the production of chromium by thepresent process.

Chromium boride comprising 17.32 weight percent boron was pelletized,placed in graphite crucibles and heated in vacuo at a starting pressureof about 0.05 micron in a furnace from which substantially all the airhad been removed. The following table is a compilation of thetemperature, time, average pressure during the process and an analysisof the resulting product.

What is claimed is:

1. A process for producing at least one of the metals selected from thegroup consisting of manganese and chromium from solid state borides ofsaid metals comprising charging at least one of said borides ofs saidselected metals in particulate form ranging from about 0.125 to about0.5 inch in diameter into a vacuum furnace; evacuating substantially allthe gas reactive with said boride and said selected metal from saidvacuum furnace; heating said boride of said selected metal in vacuo to atemperature sufiiciently high to produce metal vapor of said selectedmetal at least at a vapor pressure in excess of the ambient pressurewithin said vacuum furnace; maintaining a temperature sufiiciently highto maintain a metal vapor of said selected metal at least at a vaporpressure in excess of the ambient pressure within said vacuum furnacewhile essentally maintaining said boride in said solid state; conductingat least a portion of said metal vapor to a condensing zone andcondensing said metal vapor to form at least one of said selected metalsin substantially pure metallic form.

2. A process for producing manganese from manganese boride comprisingcharging manganese boride in the solid state in particulate form rangingfrom about 0.125 to about 0.5' inch in diameter into a vacuum furnace,evacuating substantially all the gas reactive with said manganese borideand said manganese; heating said manganese boride in said solid state invacuo to within the range of about 1500 C. to 1750 C. to producemanganese vapor; maintaining said temperature within said range tomaintain a metal vapor of said selected metal; conducting at least aportion of said manganese vapor to a condensing zone and condensing saidmetal vapor to form manganese metal in substantially pure form; andrecycling at least the boron remaining in the residue to produceadditional borides.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR PRODUCING AT LEAST ONE OF THE METALS SELECTED FROM THEGROUP CONSISTING OF MANGANESE AND CHROMIUM FROM SOLID STATE BORIDES OFSAID METALS COMPRISING CHARGING AT LEAST ONE OF SAID BORIDES OF SAIDSELECTED METALS IN PARTICULATE FROM RANGING FROM ABOUT 0.125 TO ABOUT0.5 INCH IN DIAMETER INTO A VACUUM FURNACE; EVACUATING SUBSTANTIALLY ALLTHE GAS REACTIVE WITH SAID BORIDE AND SAID SELECTED METAL FROM SAIDVACUUM FURNACE; HEATING SAID BORIDE OF SAID SELECTED METAL IN VACUO TO ATEMPRATURE SUFFICIENTLY HIGH TO PRODUCE METAL VAPOR OF SAID SELECTEDMETAL AT LEAST A VAPOR PRESSURE IN EXCESS OF THE AMBIENT PRESSURE WITHINSAID VACUUM FURNACE; MAINTAINING A TEMPERATURE SUFFICIENTLY HIGH TOMAINTAIN A METAL VAPOR OF SAID SELECTED METAL AT LEAST AT A VAPORPRESSURE IN EXCESS OF THE AMBIENT PRESSURE WITHIN SAID VACUUM FURNACEWHILE ESSENTIALLY MAINTAINING SAID BORIDE IN SAID SOLID STATE;CONDUCTING AT LEAST A PORTION OF SAID METAL VAPOR TO A CONDENSING ZONEAND CONDENSING SAID METAL VAPOR TO FORM AT LEAST ONE OF SAID SELECTEDMETALS IN SUBSTANTIALLY PURE METALLIC FORM.